Method of making tungsten filaments



Dec. 18, 1962 J. w. FRAZER METHOD OF MAKING TUNGSTEN FILAMENTS Filed July 29, 1959 INVENTOR JACK W. FRAZER am ws ATTORNEY.

United S tates Patent Ofifice 3,069,584 Patented Dec. 18, 1962 3,66%,584 METHUD 6F MAKENG TUNGSTEN FILAMENTS Black W. Frazer, Livermore, (Ialifi, assignor to the United States of America as represented by the United States Atomic Energy (Jommission Filed Jinly 2.9, H59, Ser. No. 836,440 2 Ciaims.- (Cl. 3l3--34I) This invention relates in general to tungsten filaments and a method of producing same. More particularly it relates to a tungsten filament having a tantalum core, and to a method of producing same.

The invention provides a tungsten filament capable of use in a mass spectrometer for measuring minute amounts of isotopes which have mass weights approximately the same as the weights of potassium polymers, thorium isotopes and/or other impurities presently found in commercially available filaments. The process and product of the invention eliminate these impurities. Such filaments are produced in a rate limiting process in which tantalum is evaporated from a strip core while potassiumfree tungsten is deposited thereon by simultaneous decompositions of tungsten carbonyl vapors. The filament comprises a thin core of tantalum having an outer layer of tantalum diffused tungsten bonded thereto.

Tungsten filaments are Widely used in mass spectrometers to heat and vaporize solid samples which have low vapor pressures, e.g., most compounds containing elements of high atomic weight. In such measurements, the solid sample is generally first made to adhere to a filament which is then disposed within containment means adjacent to'a tungsten heating filament. Upon heating of the heating filament, heat is also induced into the samplebearing filament so that the sample is vaporized. Thereafter the sample vapors pass by thermal motion into a region where they are ionized and the ions are thereafter subjected to forces which separate them according to their masses, all of the above being Well known in the art.

A problem encountered in measuring minute quantities of heavy element isotopes of masses of 234 to 245 with mass spectrometers is that impurities in the tungsten filaments having mass weights in the same range as contain heavy elements are also vaporized. The most important or offensive of these impurities are believed to be the polymer ions of potassium. This identification is based upon the use of potassium in the metallurgy of tungsten and on the occurrence of the ions at the correct mass numbers and with the correct isotope ratios up to K-,*', although the invention is not in any way contingent upon this identification. Other impurities having relative mass weights in the 240 range are also known to be present, i.e., thorium isotopes.

No method has been known in the prior art by which a tungsten filament completely free from such interference peaks can be easily produced from commercially competitive materials. Such impurities are present in commercially available tungsten, and potassium salts are generally added to tungsten before forming into a conventional commercial filament in order to increase its ductility and to decrease brittleness and failure at high temperatures. The impurities cannot be removed completely by baking or by other normal procedures. Graphite and tungsten carbide filaments have been impractical, as have other high temperature materials, for the used mentioned, where the heating temperature is 26002800 C. Attempts to use tantalum by itself have proven impractical because of its high vapor pressure at temperatures above 2500" C.

A mass spectrograph sample filament has now been invented which may be used to measure heavy element isotopes of masses between 234 and 245 in barely detectable amounts without the troublesome interferences with impurities present in previously available filaments. The

filament comprises a tantalum core generally less than 1 -mil in diameter having a coating of potassium-free tantalum-diffused tungsten molecularly bonded thereto. In the preferred process of manufacture a short, thin tantalum filament is first mounted between terminal posts mounted in insulated relation through a backing plate. The tungsten is most conveniently vapor plated onto the tantalum by a tungsten carbonyl vapor decomposition method having a step which is critical because of the tendency of the tantalum to volatilize at the temperature of operation of the filament. The preferred recipe comprises volatilizing tantalum by resistance heating until the current drops by about 40%, cutting the voltage back to build up the tungsten, and then gradually building the temperature back up to balance the rate of tungsten deposition with the rate of tantalum volatilization. Used in a mass spectrometer which is capable of measuring less than micro-micrograms of uranium, the background between masses 230 and 245, and in particular at masses 234 and 236 (the two largest potassium polymer peaks in the region) is undetectable. Although in spectrographic use the tantalum diffuses to an appreciable extent through the tantalum-diffused tungsten layer, particularly at sample temperatures ranging as high as 2800 C., this does not interfere with the analytical measurements mentioned. The life of the filament compares favorably with that of pure tungsten filaments.

Accordingly, an object of the invention is to provide a mass spectrographic sample filament for measurement of small amounts of materials composed of elements having high Z numbers, specifically isotopes having masses between 230 and 245, and a method of producing same.

Another object of the invention is to provide a sample filament for mass spectrographic work having a thin strip core of tantalum onto which has been bonded a coating of tantalum-diffused tungsten, the filament being mounted between terminal posts in insulated relation to a backing plate. Only tungsten and tantalum free from potassium and other impurities having mass spectrographic mass peaks in the region of interference are used in producing the filament.

Another object is to provide a method of producing a tantalum-diffused tungsten coating free from impurities on a thin tatalum substrate by tungsten carbonyl deposition.

A further object is to provide a critical resistance heating step therefor in which tantalum volatilization is balanced by tungsten deposition to provide a filament of the desired cross sectional dimensions.

The invention will be better understood upon reference to the following description and figures, of which:

FIGURE 1 is a top view of a filament mounted between two terminal posts in a backing plate,

FIGURE 2 is a side view of the entire assembly, partly in cross section along line 2-2 of FIGURE 1, and

FIGURE 3 is a cross section view of the filament along line 3-3 of FIGURE 2.

In the practice of the invention a thin elongated tantalum filament is first mounted in any manner suitable for use as a sample filament for mass spectrographic use, e.g., on a backing plate. The tantalum must be free from interfering impurities, such filaments being commonly available. The assembly is then disposed in a closed tunghsten carbonyl deposition chamber in which the filament is wired for resistance heating, any of a number of embodiments known in the prior art being suitable. The coating is applied by deposition under the equilibrium conditions at which the filament will be used, namely, temperatures at which the tantalum diffuses into the molecu larly bonded tungsten coating and to some extent vaporizes. In producing the coating the filament heating time, the temperature of the filament and of evaporation of carbonyl, and the evacuating pressure are all variably regulated. Specifically, the chamber is evacuated to about one micron and the filament temperature gradually raised to a temperature dependent upon the width of the filament, but generally above 2000 C. The tantalum begins to volatilize but as the temperature is raised tungsten free from impurities is simultatneously deposited thereon. As more tungsten is deposited the rate of tantalum vaporization is depressed. There results a tantalum strip core having a molecularly bonded coating of tantalum-ditfused tungsten generally thicker than the cross sectional width of the resulting tantalum strip core.

In the preferred embodiment of FIGURES 1-3 filament 11 is spot welded at 12 to inwardly chamfered ends if of two terminal posts-14 bent toward the plane of backing plate 15 in generally parallel alignment. In practice tantalum filaments having an initial thickness of 0.5-1.0 mil and a width of 30 mils have most frequently been used. Final thickness is somewhat greater. Distance between posts is commonly one-quarter of an inch or more. Additional strength is achieved by also welding braces 16 of similar material to the inside of filament 11. The posts must be cap-able of withstanding reasonably high temperatures and should therefore be fabricated from thermally stable material such as 30 mil Kovar wire. Kov-ar wire consists of iron containing 20 Weight percent nickel, 16 weight percent cobalt and 0.2 weight percent manganese; Insulation from the backing plate is provided by glass seal 1'2 extending through bores 18. After the coating step, the filament cross section at a central point as shown'in FIGURE 3 has a central tantalum core 21 surrounded by tantalum-diffused tungsten 19.

l'n-the detailed steps ofthe process of the invention'for producing. the tantalum filament coated with tantalumdiffused tungsten, the filament is contacted with tungsten 1 carbonyl vapors in such a manner as to cause deposition thereon of tungsten as practiced previously in the art. However, unlike the prior art, the substrate is at the same time volatilized in a critical heating sequence; Specifically, the filament assembly of the invention, prior to coating, i.e., prepared as described hereinabove, is disposed in a chamber capable of use for vapor deposition of tung-- sten from tungsten carbonyl. The chamber is evacuated preferably through a port to a pressure in the 1 micron Hg range and the filament, or terminals thereof, are attached therein by means of sealed leads to a variable current source. Normally the chamber is remotely opened to an isolated solid tungsten carbonyl source so that pure tungsten carbonyl vapors are introduced into the coating chamber by the vacuum pump pumping against the solid' source. The coating chamber is exteriorly heated, as by containment within a large adjustable furnace or by resistance wire tape thereabout, and maintained at a constant temperature;

While the chamber is held at a constant low pressure and at a constant temperature somewhat above room temperature the temperature of the filament is raised and the tungsten carbonyl vapors allowed to come into contact therewith. The time temperature sequence is carefully controlled. Initially the tungsten commences to decompose onto the filament, most heavily in the middle, leaving a deposit thereon. As the temperature is increased, however, until a thick coat is established, the tantalum tends to diffuse outward through the tungsten and vaporize. As the temperature is increased the ends of the filament away from the center point also are heated to higher temperatures and tungsten is deposited thereon. Since the background of tantalum is lowest in mass spectrographic work when the tantalum strip core is thinnest, it is generally preferable to initially vaporize a portion of the tantalum strip before initiating tungsten deposition. Coating growth is measured by correlation with carbon dioxide evolution, the latter being determined by collecting the carbon dioxide in a gas buret connected to the pump exhaust. The coating growth rate itself can be controlled 4 easily by regulating the temperature of the tungsten carbonvl source.

The process is continued until the tantalum-diffused tungsten coating is at least one-half mil thick. The resulting filament consists of a thin strip core of metallic tantalum having an outer coating of metallic tantalumdiifused tungsten of variable width bonded thereto by an intermolecular bond. in practice, where the resulting filament is 1 /2 mils thick at a point centrally between the terminal posts, the amount of tantalum remaining is about 10% of that width.

A preferred method of producing the tantalum-diffused tungsten coating is to mount a 0.5 mil thick tantalum filament between backing posts as described hereinbefore. With the carbonyl vapors at a low pressure in the deposition chamber the filament is heated therein until a substantial portion of the width across a central cross section has been removed by vaporization. A temperature above 2000 C. suffices to effect this reduction. The temperature is then reduced by means of current regulation to a temperature below which there is vaporization of the tantalum, yet at which carbonyl decomposition takes place, e.g., below 1000 C. The carbonyl vapor pressure is then raised and the temperature of the filament is gradually increased to a point somewhere in the 2000 C. range, the precise temperature for any given coating being dependent upon the rate at which the coating has built up and the thickness of the residual strip core. Where thicker tantalum strips are used, or when the tantalum is not initially vaporized almost to the point of failure of the filament, the final temperature may reach the range of 26002800 C.

Example A plurality of tantalum filaments, each 0.5 or 1.0 mil thick and mils wide, were mounted between Kovar terminal posts held by insulated backing plates about one-half inch apart as specified in the specification hereinabove. The assemblies were placed in a conventional carbonyl deposition chamber equipped with sealed leads to a variable current source in individual experiments. The temperature of the chamber was maintained at 25 C. to 54 C. and the initial vacuum pressure was 1 micron.

Initially the 0.5 mil thick filaments were heated at constant voltage until the current dropped by about in an atmosphere of tungsten carbonyl vapors at low pressure (i.e., 25 C.). Tungsten carbonyl vapors from an isolated solid source were then increased in pressure and held at 35-54 C. and the voltage was lowered sufiiciently to favor a rate of tungsten deposition greater than tantalum vaporization, i.e., a temperature lower than 1000 C. As more tungsten was deposited the temperature Was raised to the 2000 C. range over a period of 30 to minutes. There resulted in each instance a filament about 1 to 1 /2 mils thick at the center point and having a strip core of tantalum at that point about 0.2 mil thick. Life of the filaments in mass spectrometer determinations averaged that of commercially available filaments. When run or tested without a sample or prior use on the mass spectrometer no background mass peaks between 230 and 245 were noted within the limit of error of the mass spectrometer, namely 100 micro-micrograms.

The tantalum filaments 1.0 mil thick were exposed to the carbonyl vapors as the temperature was gradually raised from 1900 C. initially to a temperature in the 2600 to 2800 C. range over a period of 30 minutes to minutes, at which time the filament was completed. There resulted a filament comparable in quality with that produced from the thinner tantalum strip, although more subject to failure during the manufacture thereof because of the higher temperatures of disposition.

While there has been described in the foregoing what may be considered to be preferred embodiments of the invention, modification may be made therein without rom the teachings of the invention and it nded to cover all such as fall Within the scope of e appended claims.

What is claimed is:

1. In a filament for heating solid samples of heavy elements in mass spectrographic determinations, the combination comprislng a tantalum core having an outer coating of tantalum-diffused tungsten at least one-half mil thick molecularly bonded thereto by contacting an initial tantalum filament with pure tungsten carbonyl vapors while raising the temperature of said tantalum filament toward 2600 C. over a period of 30 to 90 minutes, said tungsten and said tantalum being free of impurities having mass numbers in the range of 230 to 245 in a mass spectrograph capable of analysis in the 100 micro-microgram range, whereby said tantalum partially vaporizes and said tungsten simultaneously deposits thereon to form said molecularly bonded coating.

speaeaa 2. The filament of claim 1 in which said initial tantalum filament is less than 1.5 mils thick. and is Welded to wire posts which are at least 30 mils thick and supported in insulated relation from a backing plate, said Wire post being composed of iron containing 20 percent nickel, 17 percent cobalt and 0.2 percent manganese by weight.

References Cited in the file of this patent UNITED STATES PATENTS 2,347,501 Penter Apr. 25, 1944 2,438,732 Williams Mar. 30, 1948 2,820,920 Penon Jan. 21, 1958 2,827,587 Longacre Mar. 18, 1958 2,867,032 Gehrke et al. Jan. 6, 1959 2,879,431 Longacre Mar. 24, 1959 

1. IN A FILAMENT FOR HEATING SOLID SAMPLES OF HEAVY ELEMENTS IN MASS SPECTROGRAPHIC DETERMINATIONS, THE COMBINATION COMPRISING A TANTALUM CORE HAVING AN OUTER COATING OF TANTALUM-DIFFUSED TUNGSTEN AT LEAST ONE-HALF MIL THICK 